Zirconia (zirconium dioxide) finds use as a catalyst or as a carrier or support material for a catalyst. When being used either as a catalyst or a catalyst carrier, it is desirable that the zirconia has a high surface area and that the surface area is stable at high temperatures. Further, to be suitable for use as a catalyst or catalyst carrier in many applications, it is also important that the zirconia product has sufficient strength for the required application. This is particularly the case when the catalyst comprises particles of zirconia which have been shaped, for example by extrusion.
Zirconia exists in a number of crystalline forms, dependant upon the prevailing conditions. Thus, zirconia exists under ambient conditions of temperature and pressure as a stable, monoclinic crystalline structure. Under extreme pressures or at higher temperatures, typically of the order of 450.degree. to 1000.degree. C., zirconia exists as a tetragonal crystalline structure. At even higher temperatures, typically in excess of 1500.degree. C., a cubic crystalline phase forms. For a general discussion of the properties of zirconia, reference is made to Kirk-Othmer "Encyclopedia of Chemical Technology", Second Edition, Volume 22, pages 651 to 655.
The preparation of zirconia may be achieved by methods well known in the art. Thus, French patent application publication No. 2 590 887 (FR-A-2 590 887) discloses the preparation of zirconia by a method comprising precipitating zirconium hydroxide from a solution of zirconyl nitrate by the addition of aqueous ammonia solution, followed by drying and calcining the resulting precipitate. Similar methods are disclosed by B. H. Davis et al "Catalytic Conversion of Alcohols. II. Influence of Preparation and Pretreatment on the Selectivity of Zirconia", Ind Eng Chem Prod Res Dev., vol 18, No 3, 1979, pages 191 to 198, and M. J. Torralvo et al, "Crystallisation Behaviour of Zirconium Oxide Gels", Journal of Catalysis 86 (1984), pages 473 to 476. U.S. Pat. No. 4,822,575 discloses a similar method in which zirconia is prepared by the calcination of the precipitate formed upon the addition of ammonia to an aqueous solution of zirconium sulphate.
P. D. L. Mercera et al, "Zirconia as a Support for Catalysts: Evolution of the Texture and Structure on Calcination in Air", Applied Catalysis, 57 (1990) pages 127 to 148, describe the preparation of zirconia samples by precipitation from a solution of zirconyl chloride at a pH of 10, followed by calcination in air at temperatures of up to 850.degree. C. This gel-precipitation technique yielded zirconia having a high surface area (up to 111 m.sup.2 /g measured by BET after calcination at 450.degree. C). The zirconia had a well developed mesoporous structure. However, P. D. L. Mercera et al report that the porous texture was unstable, with the initial high surface area being lost rapidly with increasing calcination temperature. The gel prepared during the experiments crystallized upon calcination into the tetragonal structure. However, P. D. L. Mercera found that, upon cooling, the zirconia underwent a phase transformation, yielding a substantially monoclinic crystalline product, resulting in a loss of surface area and pore volume. This in turn resulted in shrinkage of the zirconia structure and a reduction in the material's strength.
A number of methods have been proposed for preparing stable, high surface area zirconia. Thus, USSR patent number 1 370 079 (SU-A-1 370 079) discloses a process for the preparation of monoclinic zirconia comprising the reaction of an aqueous ammonia solution with an aqueous solution of zirconium nitrate, washing and drying the gel so-obtained, and then subjecting the product to a heat treatment in which it is held at a temperature of from 150.degree. to 175.degree. C. under a pressure of 6 to 10 atmospheres in the presence of steam for 2 to 10 hours. It is stated in SU-A-1 370 079 that the process yields zirconia free from tetragonal crystalline phase, having a high specific surface area of 105 m.sup.2 /g and a pore volume of 0.19 ml/g.
Further, FR-A-2 590 887 referred to hereinbefore discloses a composition comprising zirconia in combination with an additive selected from an oxide of silicon, a rare earth metal or aluminum. The additive is said to stabilize the specific surface area of the zirconia at high temperatures. Preferred stabilizing additives are yttrium, lanthanum and cerium. The additive may be present in the composition in an amount of from 1 to 10% by weight, preferably from 2 to 5% by weight. The composition may be prepared by the co-precipitation of a precursor of the additive compound and a precursor of zirconia. Alternatively, the additive may be incorporated into the composition by the impregnation of a zirconia material.
U.S. Pat. Nos. 2,442,772 and 2,467,089 (US-A-2,442,772 and US-A-2,467,089, respectively) both disclose a method for the preparation of stable zirconia hydrogels. The method comprises reacting an aqueous solution of a soluble zirconium compound with water soluble salts of at least two weak acids, such as acetic acid, carbonic acid and nitrous acid, to yield homogeneous zirconia hydrosols containing substantially no precipitate. US-A-2,442,772 and US-A-2,467,089 describe using the product as such as a catalyst or catalyst component. Alternatively, the product may be impregnated with other oxides or the like or milled with other materials. The milling may be carried out with the hydrogel in a wet form, optionally with wet silica gel, in a ball mill, prior to extrusion.
As mentioned hereinbefore, many catalytic applications employ preshaped catalyst particles, shaped using such techniques as extrusion and pelletizing. European patent application publication No. 0 510 772 (EP-A-0 510 772) discloses a process for the preparation of a zirconia-based catalyst or catalyst precursor comprising mulling a mixture of zirconia and/or a zirconia precursor and a solvent, which mixture has a solids content of from 20 to 60% by weight, and extruding the mixture. The resulting extrudates may be impregnated with a suitable catalytically active element, preferably cobalt.
It has been found that zirconia extrudates prepared using the method described in EP-A-0 510 772 can suffer shrinkage upon calcination, leading to weaker extrudates which may not meet the specifications of strength necessary for some applications. In such cases, a refractory oxide, such as silica, may be added to the mixture being extruded to act as a binder and improve the strength of the final calcined extrudates.
It has now been found that a zirconia precursor most suitable for preparing zirconia-containing catalysts or catalyst precursors may be prepared by contacting a solution of an acidic zirconium compound with a solution of a basic zirconium compound, yielding the precursor in the form of a material hereafter referred to as a "gel".